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Abstract

This chapter describes the Grenville orogen as it is preserved in areas of outcrop as well as in the subsurface in the United States, Late Proterozoic continental rifting that fragmented that orogen, and Precambrian rocks within terranes accreted to the rifted eastern and southern margins of Laurentia (earliest Paleozoic North America). The accretion of terranes to the eastern and southern margins of Laurentia formed the Paleozoic Appalachian (Caledonide)-Ouachita orogen. Outliers of the Grenville orogen, variously deformed by Paleozoic orogenies, crop out within the western part of the Appalachian orogen.

Although protoliths as old as Archean have been identified along the northwestern margin of the Grenville orogen in Canada, as far as we know, no rocks in the areas covered by this chapter are older than Middle Proterozoic. Some rocks, however, indicate ties to older source areas. Quartzites from the Adirondack Lowlands contain detrital zircons with minimum ages of 1.65 Ga, 1.8 Ga, and 1.95 Ga (McLelland and others, 1988a). Recent work by Dunning and Cousineau (1990) and Olszewski and others (1990) on rocks of the Chain Lakes block, Maine-Quebec, and correlatives to the north in the Canadian Appalachians has shown that some zircons, possibly detrital, in diamic-tite are as old as 2.8 Ga. Sinha and Bartholomew (1984) report a discordia intercept of 1.87 ± 0.2 Ga for zircons, probably detrital, from layered gneiss of the Blue Ridge Grenvillian outlier in Virginia. Zartman and Hermes (1987) report a Late Archean inheritance in zircon from Permian granites in the southeastern New England Avalonian terrane; they attribute this to the under-plating of the Avalonian microplate by Archean crustal components, possibly of Africa, in the late Paleozoic during the collision of Gondwanaland with Avalonia.

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